As a liquid crystal optical shutter, there has conventionally been used nematic liquid crystals, such as twisted nematic liquid crystals and super twisted nematic liquid crystals. These nematic liquid crystals, however, require a polarizer. Several troublesome problems are associated with the use of a polarizer. For example, optical transmissivity is poor, and viewing angles are narrow. Further, the thickness of a nematic liquid crystal cell must be controlled to a highly precise degree, which imposes great difficulty to the enlargement of the area of a liquid crystal cell.
In the few last years, research and development efforts have been directed to overcoming these problems and to liquid crystal/polymer composite systems, the so-called polymer-dispersed type, which are optical shutter devices capable of providing large liquid crystal displays with low optical loss and low cost production.
Most of the optical shutter devices of liquid crystal/polymer composite systems developed so far employ nematic liquid crystals. In this connection, there are two main types of liquid crystal/polymer composite based on their morphology. One type is a structure of dispersed droplets of liquid crystal in a polymeric matrix, being referred to as polymer dispersed liquid crystal (PDLC) (see J.W. Doane, N. A. Vez, B. G. Wu, S. Zumer [1986] Appl. Phys. Lett. 48:27). The other type is a structure of three-dimensional networks or microdroplets of polymer in a continuous phase of liquid crystal. This second type is referred to as polymer network liquid crystal (PNLC). This is described in Japanese Patent Laid-Open Publication Nos. Heisei 2-28284 and Heisei 2-55318.
However, liquid crystal/polymer composite optical shutter devices based on the nematic liquid crystals are considerably slow in response times and demand relatively high voltages for operation, compared with other optical shutter devices.
An attempt was made not only to enhance response time but also to improve contrast ratio in the liquid crystal optical shutter of the polymer-dispersed type, by using chiral nematic liquid crystals containing 5 to 10% by weight of chiral dopant and giving helical twisting power to the liquid crystals (see H. Fujitake et al. [1990] NHK Kiken, The 16th Discussion for Liquid Crystal, pamphlet, p120). According to this described method, significant improvement in response time can be effected. However, the voltage necessary to the operation becomes high, which leaves problems to be solved for its practical use.
Another attempt was made to lower the operating voltage without the delay of response time, by using a small quantity of a mixture of two different chiral dopants and lowering the concentration of the polymer (see P. Jiang and T. Asada [1992] Mol. Cryst. Liq. Cryst. 222:87). In this case, the optical transmissivity, however, reaches only 70% at an applied voltage of 6 V. Also, the use of polymethylmethacrylate, a polymer showing a glass phase at room temperature, causes the optical shutter to be vulnerable to impact. Accordingly, many problems remain.